3D city models are getting more important as a field of research and business and received an increasing amount of attention from both the scientific community and the professional field. 3D city models are one of the new tools for sustainable city development. Since development questions occur repeatedly, the used city models should be maintainable, i.e. the system should be kept up-to-date and not be created new for each decision. This is a challenge. Creating a model representing the current status of a city has been addressed in research literature. The major challenge is the vast amount of data to be collected, processed and visualised. However, keeping the resulting model up-to-date has not been discussed yet. Updating requires the introduction of a suitable concept of time in the model. This would then allow representing historic and current status of the city as well as future scenarios. Processes provide the connection between different points in time. Processes also change the appearance of the city and need to be represented in the model for change detection. In this paper, we discuss the challenges and show necessary properties for city models and systems maintaining them to reach a reasonable level of maintainability.